(1) Field of the Invention
The present invention relates to a radiographic image processing method wherein space-frequency processing of a radiographic image of an object, typically a human body is carried out.
(2) Description of the Related Art
Conventionally, there are two methods for diagnosing a human body by radiography. In one method, X-rays transmitted through a human body are applied to a fluorescent screen to transform the X-rays to visible rays, then the visible rays are applied to a silver film to form a latent image, and the latent image is developed to obtain a radiograph. The radiographic image is photo-electrically read by a film reader device for electronically processing the radiographic image to improve the quality of the radiographic image.
In the other method, X-rays transmitted through a human body are applied to an accelerated phosphorescent (which may be called photo-stimulable phosphor, or storage phosphor) plate to store a radiographic image therein, and then the radiographic image is photoelectrically read from the accelerated phosphorescent plate by applying excitation rays thereto for electronically processing the radiographic image to improve the quality of the radiographic image. The above method using the accelerated phosphorescent plate is disclosed in the U.S. Pat. No. 5,859,527.
However, for example, when the conventional radiograph of the chest is processed so that the contrast in an image of a lungfield is good, only images of blood vessels are visible as low density portions in the lungfield of a high density, but images of other portions such as a mediastinum, a diaphragm, and a body of vertebra, are not visible because of very low density thereof. Otherwise, when the conventional radiograph is processed so that contrasts in images of portions other than a lungfield are good, the images of blood vessels in the lungfield are not visible because of a very high density thereof. This is because the dynamic ranges of human eyes, silver films, and the accelerated phosphorescent plate.
Generally, diseases may occur in every portion of a human body, and therefore, it is desirable that every portion in a radiographic image is sufficiently visible in a radiographic image for a person diagnosing the human body.
In the conventional method for processing a radiographic image, image data S of each pixel (i, j) in a radiographic image containing l.times.m pixels is transformed to a processed image data Q as follows, where i=1 to l and j=1 to m. An average of image data of an averaging area containing l.times.m pixels including the pixel (i, j) in the center thereof. The image data S of each pixel (i, j) in a radiographic image containing l.times.m pixels is transformed to a processed image data Q in accordance with the following relationship, EQU Q=S+K.multidot.(S-S.sub.m) (1)
where K (K&gt;1) is a constant for enhancing a high space-frequency component in a radiographic image. Here, a degree of enhancement is defined as .vertline.Q-S.vertline.. In accordance with the equation (1), the degree of enhancement .vertline.Q-S.vertline. is expressed as, EQU .vertline.Q-S.vertline.=K.multidot..vertline.S-S.sub.m .vertline..(2)
FIG. 1 is a diagram indicating space-frequency characteristics of an original image data S, an average S.sub.m, a processed image data Q, and a difference between the original image data S and the average S.sub.m. As indicated in FIG. 1, the difference S-S.sub.m between the original image data S and the average S.sub.m is a component wherein a high space-frequency component is enhanced. Namely, the above transformation in accordance with the equation (2) enhances the high space-frequency component in the original image data S.
FIG. 2 is a diagram indicating a relationship between original pixel values S of the image data and processed pixel values Q of the processed image data, and a relationship between the original pixel values S of the image data and the degree of enhancement .vertline.Q-S.vertline., where the original pixel values S vary from 0 to 1023, and the average S.sub.m is assumed to be 511. As indicated in FIG. 2, according to the above conventional method for processing a radiographic image, the degree of enhancement .vertline.Q-S.vertline. becomes a very large value when the difference S-S.sub.m between the original image data S and the average S.sub.m is large. Namely, according to the above method, a portion which originally seems to be a sharp edge is transformed to be an excessively sharp edge. When a doctor inspects a radiographic image, the attention of the doctor tends to be attracted to the sharp edge, and a small variation of the density in the radiographic image tends to be overlooked by the doctor, while the image of a lung tumor may appear in a lungfield as a area of density which is a slightly smaller than the density of a sound lungfield. That is, the lung tumor tends to be overlooked by the conventional processing method as explained above.
To improve the above drawback, the Japanese Unexamined Patent Publication No. 56-104645 discloses a technique wherein image data S of each pixel in a radiographic image is transformed to processed image data Q in accordance with the following relationship, EQU Q=S+g(S-S.sub.m) (3)
where g(S-S.sub.m) is an odd increasing monotonic function of the difference between the original image data S and the average S.sub.m. Since the function g(S-S.sub.m) used in the equation (3) varies in accordance with an increasing monotonic function as indicated in FIG. 3 as an example, the drawback that a small variation of the density in the radiographic image tends to be overlooked by the doctor because the attention of the doctor is attracted by the other sharp edge, is relieved slightly but not sufficiently.
In addition, since the above function g(S-S.sub.m) used in the equation (3) is an odd function, the degree of enhancement .vertline.Q-S.vertline. is the same whether the difference S-S.sub.m between the original image data S and the average S.sub.m is larger or smaller than zero.